The invention generally relates to a work size checking apparatus capable of continuously checking the sizes of the works, depending upon special properties of the works. More specifically, the invention is directed to such a work size checking apparatus that an allowable range is set for size errors which have been precisely patterned based upon either a maximum value or a minimum value which are obtained by overwriting measurement values of a work or a standard model. Alternatively, the allowable range can be based on upper limit value and a lower limit value which are obtained by rearranging an average value of the measurement values by a standard deviation at an arbitrary measuring point. The allowable error range is then overwritten by actually measured values of a work for comparison purposes.
In an IC (integrated circuit) manufacturing process, a test is performed by checking the sizes of the heights of leads employed in the ICs. The ICs are not operable when the ICs, having incorrect heights of the leads, are mounted on printed circuit boards and loose contacts occur.
As the conventional size checking apparatuses, it has been proposed that the lead portions of an IC correctly positioned on a reference plane are imaged by a CCD camera. The image data acquired by the CCD camera are processed in a computer in order to check the conditions of the leads, and further heights of the leads of the IC which have been precisely positioned on a reference plane are measured as numerical values by employing a laser displacement gage.
However, these conventional size checking apparatuses have various problems. To achieve correct size judgments, the IC must be precisely positioned at a preselected location, therefore an apparatus for precisely positioning the IC is required. Furthermore, when this positioning apparatus is operated over a long time period, mechanical damage occurs, such as deformation and abrasion of various components. Correct size judgments are difficult to make, resulting in another problem. Additionally, it is a very cumbersome task to confirm how the positioning precision by the positioning apparatus is degraded and/or to what extent the restoring work has been carried out.
Also, in the conventional size checking apparatus, the allowable measurement range defined by the upper limit and the lower limit which have been set as a measure of allowable size errors is not preset based upon a large quantity of measurement values. For instance, it takes a large amount of measuring time to investigate whether or not the sizes of leads employed in an IC, that which are presently transported on the manufacturing belt line are within the allowable range. Since the ICs have specific structures that cannot be measured (e.g. leads located inside the IC housing), both the upper and lower limit judging standard values must be collected by rechecking the IC products which have been judged defective based on the design clearance.
Moreover, since the conventional size checking apparatus has a construction such both the upper limit value and the lower limit value, which are in conformity to a target fraction defect defined by the standard deviation or the like, cannot be simply set, the checking efficiency cannot be quickly increased.
The conventional size checking apparatus has a further problem that, because it is difficult to confirm which portions of leads the laser beam of the laser displacement gage is directed toward, a user may be afraid to rely on the checking results.
In addition, there is a further problem in that the positioning apparatus, the CCD camera, and the laser displacement gage must be of a high precision design so as to perform a zero point correction and to achieve the desired precision. As a result complicated and difficult adjustments are required.